If 1,800 mSv/h — logged a few days earlier at a different spot near the same tank — was high enough to kill a person in four hours, as operator Tokyo Electric Power Co. confirmed at the time – then we’re looking at a truly deadly situation at the site of the world’s worst nuclear accident since Chernobyl, as many in the blogosphere have said.

JRT’s bottom line, which we’ll explain with a quick primer on the latest levels, is that they’re lethal all right, but you’d probably have to twist yourself in pretzels to get a deadly dose.

First, a bit about radiation readings. The levels that Tepco announces are largely comprised of two different kinds of radiation: beta and gamma rays.

Beta radiation, emitted by strontium-90 and tritium, is weaker than gamma – meaning less energy is released – and it doesn’t travel as far. The U.S.-based Health Physics Society says on its website that beta radiation “can penetrate human skin to the ‘germinal layer,’ where new skin cells are produced. If high levels of beta-emitting contaminants are allowed to remain on the skin for a prolonged period of time, they may cause skin injury.’” Even clothing can provide some protection against beta radiation.

Gamma radiation, emitted by cesium-137 or iodine-131, packs more energy – thus causing more damage to human tissue – and can penetrate all but the heaviest materials. That’s why Fukushima Daiichi’s command center windows are shielded with lead, and the vessels containing the reactors’ nuclear cores were shrouded by several meters of concrete.

So to a person standing on a hot spot, the kind of radiation makes a big difference. In the recent Tepco readings, the vast majority has been beta radiation.

The Nuclear Regulation Authority, which says it’s been trying – unsuccessfully – for more than a week to get Tepco to release a breakdown of gamma versus beta radiation for its readings – says less than 1 mSv/h of the 2,200 mSv/h announced Wednesday is gamma radiation.

That means the spot is deadly if you’ve got your face inches away, but the danger drops rapidly if you stand up, say, or put on protective clothing.

Tepco says the 2,200 mSv/h reading, for instance, would be the exposure at 5 cm away. That drops to 40 mSv/h at 50 cm, Tepco says.

Just how bad is all that? In a largely unintelligible explanation put out a few days ago, Tepco says 1) a level of 7,000 mSv is fatal, 2) the “control level of the equivalent dose for skin” is 500 mSv/year, 3) the “annual radiation exposure limit for workers” is 50 mSv and 4) it’s “inappropriate” to compare these things with the radiation levels measured near the tanks anyway, since “the standard levels represent the cumulative effective dose (not equivalent dose) upon the whole body.”

They tell us that, although scientists have a measure, called the gray, for how much energy will be absorbed in a kilogram of matter from a radioactive source, what most people working around nuclear material want to know is: How much damage will this do to me?

Since the amount of damage depends on the type of radiation involved, the amount of energy has to be weighted by whether the source is gamma or beta rays, for instance. The result is called the “equivalent dose,” and is measured in sieverts (1 sievert = 1,000 millisieverts).

The Tepco radiation measurements from leaky tanks – like the 2,200 mSv/h reading – is an “equivalent dose” reading.

What’s more, different types of tissue sustain different amounts of damage from the same amount of radiation – the unprotected eye, for instance, is more vulnerable than a tough organ like skin. So Japan limits equivalent dose exposure of the skin to 500 mSv per year, but curbs exposure of the lens of the eye to 150 mSv per year.

What if you wanted to know the overall impact of radiation on your body? That’s where the “effective dose” comes in. It uses a formula to weight the impact of radiation on each bodily tissue, and averages it out to come up with a single number – also measured in sieverts – for overall exposure. Japan limits (male) nuclear workers to an effective dose exposure of 100 mSv for five years, or 50 mSv per year.

How harmful is 50 mSv of exposure over a year? Scientists generally say the impact likely isn’t that high. Experiments have shown the risk of cancer rising marginally, starting at 100 mSv of exposure (below that, the effects are assumed to be there, but they aren’t measurable).

So a reading of 2,200 mSv/h is surely something to worry about, but because of the type of radiation, it’s not cause for wholesale panic.

For those who remain fearful, the head of Japan’s Nuclear Regulation Authority, Shunichi Tanaka, had this to say at a press conference Monday: “Historically, we have seen much higher levels of radioactivity. For example, when I was a kid there were nuclear bomb experiments in the Pacific, there was much higher radiation in the atmosphere, and I was told to wear a hat when I went out in the rain so as not to lose hair.”

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